Vapour phase oxidation of p-xylene

Abstract

The kinetic study on the oxidation of p-xylene has been carried out.

The oxidation of p-xylene on the following two catalysts has been studied: (a) tin vanadate (b) ferric molybdate (Boreskov’s composition) Ferric molybdate catalyst was used for the first time for the oxidation of an aromatic hydrocarbon (oxidation of p-xylene).

Kinetic analysis indicates the validity of the redox model. The rate equation of the model for both the catalysts, viz., tin vanadate and ferric molybdate, is represented as:

rx=k1k2CxCO2k2Cx+Nk1CO2r_x = \frac{k_1 k_2 C_x C_{O_2}}{k_2 C_x + N k_1 C_{O_2}}rx?=k2?Cx?+Nk1?CO2??k1?k2?Cx?CO2???

The values of the rate constants for the catalyst reduction step k1k_1k1? and catalyst reoxidation step k2k_2k2? were evaluated at different temperature levels for both catalysts. It is concluded that the reoxidation of the reduced catalyst (k2k_2k2?) is the rate?controlling step, as observed by Jirue et al., Bhattacharya et al., and Mars and van Krevelen.

The various models were compared on the basis of three criteria according to the classical approach and non?intrinsic statistical method. It is concluded that the two?stage redox model (first order with respect to both reactants) is the most applicable model.

The activation energies and pre?exponential factors were evaluated for both catalysts. A break in the Arrhenius plot was observed around a temperature of 570–375°C for the tin vanadate catalyst. No break in the Arrhenius plot was observed with the ferric molybdate catalyst. The activation energy and pre?exponential factor in both the ranges 520–560°C and 380–420°C were evaluated. The activation energy for the 580–420°C range was found to be six times that for the 520–560°C range. This behaviour may be due to an unfavourable orientation of the lattice structure.

The reaction was found to follow a parallel–consecutive route.